1. Field of the Invention
This invention relates to the control of the heat absorption in a heat exchanger to maintain the temperature of the fluid discharged from the heat exchanger at a set point value. More particularly this invention relates to the control of the temperature of the steam leaving the secondary superheater or reheater of large size fossil fuel fired drum or separator type steam generators supplying steam to a turbine having a high and a low pressure unit. As an order of magnitude such steam generators may be rated at upwards of 6,000,000 pounds of steam per hour at 2,500 psi and 1,000 degrees Fahrenheit. The generic term "superheater" as used hereafter will be understood to include a secondary superheater, a reheater or primary superheater as the control system of this invention is applicable to the control of each of these types of heat exchangers.
The steam-water and air-gas cycles for such steam generators are well known in the art and are illustrated and described in the book "Steam Its Generation and Use" published by The Babcock & Wilcox Company, Library of Congress Catalog Card No. 75-7696. Typically in such steam generators, the saturated steam leaving the drum or separator passes through a convection primary superheater, a convection or radiant secondary superheater, then through the high pressure turbine unit, convection or radiant reheater to the low pressure turbine unit. The flue gas leaving the furnace passes in reverse order through the secondary superheater, reheater and the primary superheater. To prevent physical damage to the steam generator and turbine and to maintain maximum cycle efficiency it is essential that the steam leaving the secondary superheater and reheater be maintained at set point values.
It is well known in the art that the heat absorption in a heat exchanger such as a superheater or reheater is a function of the mass gas flow across the heat transfer surface and the gas temperature. Accordingly, if uncontrolled, the temperature of the steam leaving a convection superheater or reheater will increase with steam generation load and excess air, whereas the temperature of the steam leaving a radiant superheater or reheater will decrease with steam generator load.
The functional relationship between boiler load and uncontrolled final steam temperature at standard or design conditions is usually available from historical data, or it may be calculated from test data. From such functional relationship there may be calculated the relationship between boiler load and flow of a convective agent, such as flow of water to a spray attemperator, required to maintain the temperature of the steam discharged from the superheater at set point value. Seldom, if ever, does a steam generator operate at standard or design conditions so that while the general characteristic between steam generator load and temperature of the steam discharged from the superheater may remain constant, the heat absorption in a superheater or reheater and hence the temperature of the steam discharged from a superheater, will, at constant load, change in accordance with system variables, such as, but not limited to, changes in excess air, feed water temperature and heat transfer surface cleanliness.
Control systems presently in use, as illustrated and described in The Babcock & Wilcox Company's publication, are of the one or two element tpe. In the one element type a feed back signal responsive to the temperature of the steam discharged from the superheater adjusts a convective agent, such as water or steam flow to a spray attemperator. In the two element type a feed forward signal responsive to changes in steam flow or air flow adjusts the convective agent which is then readjusted from the temperature of the steam discharged from the superheter. It is evident that neither of these control systems can correct for changes in the heat absorption of the superheater caused by changes in system variables.